Everypony has their inner demons. There are things inside they're ashamed to admit, if they ever admit them at all, and sides of them they wish away with all their might. Perhaps the greatest enemy to the sanity binding the monsters within us is time, as it wears away on the threads holding us to reality…

Yes, when you live forever, you have quite some time to become mad..

This is a walk through the events of Nightmare Moon's first appearance a thousand years before the start of My Little Pony: Friendship is Magic, told through the eyes of the princesses and the nightmare themselves.